Get yourself a vert grip if you don't already have one. A Magpul MIAD gripset, they are ver comfortable. Some back up iron sights (flip up for the win), if you don't have them.
Maybe some red dot sight, or a bushnell holosight (if you are willing to spend the money).
The possibilities for an M4 are endless. It all depends on what you want. Mock silencer maybe?

Here's what I have on my M4 now: RIS (it was a Marui M4A1), vertical grip, ACOG RDS, G&P PEQ. Also I would like to replace the barrel with a one-barrel design so I can fix that stupid barrel wobble, and of course, a upper reciever. Oh one more question, what do some M4 have two flashlights if they already have a PEQ?